Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Consequently, the use of exhaust aftertreatment systems on engines to reduce emissions is increasing.
Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (e.g., diesel) engines typically monitor the release of carbon monoxide (CO), unburned hydrocarbons (UHC), diesel particulate matter (PM) such as ash and soot, and nitrogen oxides (NOx).
With regard to reducing NOx emissions, NOx reduction catalysts, including selective catalytic reduction (SCR) systems, are utilized to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, to reduce the NOx. Currently available SCR systems can produce high NOx conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from a few drawbacks.
SCR systems utilize a reductant delivery system to introduce ammonia reductant into the exhaust stream upstream of the SCR catalyst. When just the proper amount of ammonia is available at the SCR catalyst under the proper conditions, the ammonia is utilized to reduce NOx. However, if the reduction reaction rate is too slow, or if a deficient amount of reductant is introduced into the exhaust stream upstream of the SCR catalyst, the SCR system may be unable to convert enough NOx to meet regulated emission standards associated with NOx.
The reductant delivery system may under-deliver the needed amount of reductant or ammonia due to blockage of the reductant flow within the system. For example, there may be unintentional or intentional restrictions in the fluid transfer hose connecting the reductant pump with the injector of the delivery system. Alternatively, reductant may form deposits in the reductant delivery system (e.g., within the injector nozzle) and may restrict the flow of reductant through the system. Recent regulations governing SCR systems require on-board diagnostic (OBD) alerts indicating the failure of an SCR system to convert enough NOx to meet the regulated standards. One known indication of the inability of an SCR system to convert enough NOx is the inability to deliver the necessary amount of reductant for NOx conversion due to blockage within the reductant delivery system. Known systems and associated diagnostics fail to adequately diagnose poor performance of reductant delivery systems due to blockage or other failures, and thus may fail to meet the OBD requirements regarding the malfunction of reductant delivery systems.